1. Technical Field
The present invention relates to the field of digital printing, and more particularly, to surface treatments that improve digital printing.
2. Discussion of Related Art
The spreading of liquid on non-absorbing surfaces is determined by various parameters such as the surface free energy (i.e., its wetting properties), the surface morphology (i.e., smooth or porous), and the ink surface tension. For example, FIG. 1A illustrates a surface 70 having a low surface energy with respect to the applied liquid, resulting in the rejection of the liquid by the surface, as illustrated in the formation of liquid drops 85A exhibiting a contact angle θ>>90°. In case the liquid is water, the surface is considered hydrophobic. In this case, it is difficult to create a stable connection of the liquid to the surface. In another example, FIG. 1B illustrates a surface 70 having a high surface energy with respect to the applied liquid, resulting in the wetting of the surface by the liquid, as illustrated in the formation of a liquid layer 85B on surface 70, exhibiting a contact angle θ<<90°. In case the liquid is water, the surface is considered hydrophilic. In this case, it is highly problematic to control a drop and prevent it from spreading.
When printing with the same printing liquid on a multi-component surface comprising both hydrophilic porous surfaces (e.g., etched copper) and hydrophobic smooth surfaces (e.g., polyamide or glass epoxy), the drop spreading on both surfaces cannot be controlled. As illustrates in FIG. 2A, liquid drops 85A form on the hydrophobic surface while an uncontrollable liquid film 85B forms on the hydrophilic surface. In case wetting of the hydrophobic surface is improved by reducing the surface tension of the ink to be lower than the surface energy, the over-wetting of the hydrophilic surface is worsened and an unacceptably smeared and low coverage image results (smearing and low coverage are caused by the high surface energy and porosity of the hydrophilic surface). The hydrophilic surface is shown in FIG. 2A as a protrusion 71 from surface 70 to enhance lateral wetting and the fact that the edges of the metal image act as chromatographic surface and thus the capillary forces drag liquid up.
Relating specifically to digital printing on printed circuit boards (PCBs), current technologies apply the solder mask layer by spreading a viscous photoresist formulation on the PCB, partially drying the formulation, exposing the layer through a mask to radiation and etching away the exposed areas to yield the required pattern of solder mask. This process allows using a wide range of formulations and in particular using formulations of high viscosity. Such a formulation however is problematic when turning to digital printing of PCBs.